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| February 2006 a newsletter for IES customers and friends Volume 12.2 | |||
Dynamic Analysis PrimerVisualAnalysis 5.5 Mode Shapes and Response SpectraVisualAnalysis offers sophisticated dynamic analysis capabilities. What can VisualAnalysis Do?The answer to this question depends upon which level of VisualAnalysis you have purchased. The basic level does not offer any dynamic capabilities, only the static analysis. The standard level will calculate modes of vibration, as well as perform a modal-superposition based on a design spectrum (an idealized earthquake definition). The latter is called a "Response Spectrum" analysis in VisualAnalysis. In the advanced level, VisualAnalysis can also perform time-history analyses for seismic events or for vibrating machinery, but this topic will be left to a future newsletter. Seismic Analysis per IBC 2003 and ASCE 7-02Current US building codes require careful and complicated consideration of seismic forces. If you are fortunate enough to be working in a low-seismic region, or are designing buildings of low "importance", you might be able to get away with a simplified equivalent lateral force procedure [IBC 2003 1617.4; ASCE 7-02 9.5.5] or something similar. In all cases, however, you are permitted to perform more sophisticated investigation, such as the Model Response Spectrum Analysis [IBC 2003 1618; ASCE 7-02 9.5.6]. With VisualAnalysis, such an investigation might be easier and more accurate. Running a Dynamic Response AnalysisOur advanced system specialist, Jeff Lutzenberger, Ph.D., has put together a white paper on The basic steps are these:
Rigid Link Rescue!Modeling Tips & TricksIt's called the Rigid-Link, but it's really just a member element that lets you model a "geometric separation" between to elements in your model. Rigid links enforce a relationship between the displacements at two nodes in your model. Modeling is an ApproximationAs you are aware, FEA modeling is an approximation: we model real structures using mathematical simplifications, one of which is the linear element known as a member in VisualAnalysis. Member elements represent a true physical member, such as a reinforced concrete column with a line passing through the centroid of the column, and given stiffness properties to capture the behavior of the true structural entity. Mathematical SimplificationsThis leads to further simplifying assumptions, such as all members are connected at the intersections of the member's centroid. If you are modeling a 10 ft by 10 ft concrete column and a W8x10 steel beam frames into this column, connected at the face, then our "little assumption" is that the connection is made 5 ft from the actual location of the connection! Also there will be very little deflection over that 5 ft of solid concrete! Automated Rigid-Links?Rigid Links to the Rescue!Other Uses of Rigid LinksThere are actually many uses for rigid links, two of which we have discussed in the past.
There are many other situations where a little rigid link can go a long way toward simplifying your life or improving the quality of your models! Let's take a closer look at each of these last two items, as they have not been discussed here before.
Moment Released PlatesPlate elements in VisualAnalysis do not offer "moment releases" like member elements do, so all connections to and between plate elements in a frame model are "rigid" by default. If you are connecting a beam to a plate, you can use a member end release directly. However if you want to simulate a "hinged" connection between two plate meshes, there is no built-in feature to accomplish this. The solution is to separate the two meshes by a small distance, and insert short rigid link members between the two. Use a moment release at one end of each link. Here is a sample project (version 5.5) to demonstrate a plate mesh with moment releases.
Top & Bottom Lateral BracingBridge girders are often connected by horizontal trusses at the top flange. Sometimes you will also have horizontal bracing attaching to the bottom flange, or diagonal bridging between girders. Rigid links make this easy. Here is a sample project file (version 5.5) that demonstrates all three kinds of bracing and also shows how you might use the Name Filter feature to hide the rigid link members in Picture View, so that things don't look so cluttered.
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Now it's Your TurnLet the Customer Speak!VisualAnalysis 6.0 has been under development since just before the release of version 5.5 and we are working hard to build a system that "blows the doors off" any tool IES has ever created before. Much of our work is based on feedback over the years from you, and is also based on ideas we have. Who Wants What?Today, we have reached a milestone in our development cycle and it is time to ask you some questions before we move on. If you are running VisualAnalysis 5.5, please take a minute or two to complete our short survey. If you are not running VisualAnalysis 5.5, please send us an email telling us why not! Please, Pretty Please!Click the above survey link and help us, help you by answering four easy questions about VisualAnalysis 6.0. Property Line Walls"How To" in IES QuickRWallIt is not uncommon to have a retaining wall or basement wall that sits directly adjacent to a property line. In this situation your footing will be constrained to have just a toe or a heel, but not the other. You can quickly overcome this restriction in the design of your wall using IES QuickRWall for retaining wall design or checking, here's how:
On the Footing tab of the Input Criteria windows, simply enter a zero length for either the Heel length or the Toe length. Of course, the hard part will be dealing with all those little red x's:
If you have a basement wall, you will typically have a lateral support somewhere near the top of the wall. To do this in QuickRWall, just check the Has Lateral Support (Restrained Wall) option under the Stem tab of the Input Criteria windows to define the location of this support. Latest UpdatesFixes in the Olympic SpiritHey, we all fall down once in a while! Sometimes we slip, sometimes we crash, and sometimes we just get caught "showboating" a little bit! IES tools have had their share of bobbles lately, but we pick ourselves up and dust ourselves off and keep on going for Gold! Our latest releases are available on the web site along with "Change Logs" that will help you decide if you need to install the updates yet. Update Service ChangesYou may have noticed that the Check for Updates feature does not work in some IES tools, and in others the command simply takes you to our web site to download the latest release manually. We are in the process of launching a newer, simpler update service that will notify you automatically when updates are available. We appreciate your patience as we roll out these changes, with the goal of making your life easier, at least in the long run. In the mean time, you will always find our latest builds available on our web site under Download | Updates and Patches.
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Often times, engineers make it through school without ever seeing or using this kind of structural analysis, and so when the building code requires it, or when you think it might give you a competitive advantage on a certain job, you must be "self taught". While the theory may be complicated beyond a VirtualWork newsletter, we can still provide some practical tips on how to make the most of these features.
